[gdb/testsuite] Disable styling in host board local-remote-host.exp

With test-case gdb.server/stop-reply-no-thread.exp and host board
local-remote-host.exp, I run into:
...
Breakpoint 1, ^[[33mmain^[[m () at ^[[32mstop-reply-no-thread.c^[[m:21^M
21        ^[[01;34mreturn^[[m ^[[35m0^[[m^[[31m;^[[m^M
(gdb) FAIL: gdb.server/stop-reply-no-thread.exp: to_disable=: t_nonstop=off: \
  continue to main
...

The problem is that styling is enabled, and that is causing a regexp mismatch.

With native, styling is disabled in default_gdb_init by doing
'setenv TERM "dumb"', but that only has effect because the build (where we
execute runtest, and consequently the setenv) and the host (where we execute
gdb) are the same.  For this host board however, gdb executes on a remote
host, and the setenv has no effect.

We could try to make some generic way to set TERM on the host, but for the
purposes of this test-case it seems sufficient to just add:
...
set GDBFLAGS "${GDBFLAGS} -iex \"set style enabled off\""
...
so let's go with that for now.

Tested on x86_64-linux.

gdb/testsuite/boards/local-remote-host.exp

@@ -30,3 +30,5 @@ proc ${board}_spawn { board cmd } {
     set board_info($board,fileid) $spawn_id
     return $spawn_id
 }
+
+set GDBFLAGS "${GDBFLAGS} -iex \"set style enabled off\""